1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to servo writing a disk drive using a secondary actuator to control skew angle.
2. Description of the Prior Art
Disk drives for computer systems comprise a disk for storing data and a head actuated radially over the disk for writing data to and reading data from the disk. To effectuate the radial positioning of the head over the disk, the head is connected to the distal end of an actuator arm which is rotated about a pivot by a rotary actuator (e.g., a voice coil motor (VCM)). The disk is typically divided into a number of concentric, radially spaced tracks, where each track is divided into a number of data sectors. The disk is typically accessed a data sector at a time by positioning the head over the track which comprises the target data sector. As the disk spins, the head writes transitions (e.g., magnetic transitions) in the data sector to record data, and during read operations senses the transitions to recover the recorded data.
Accurate reproduction of the recorded data requires the head to be positioned very close to the centerline of the target data sector during both write and read operations. Thus, accessing a target data sector involves positioning or “seeking” the head to the target track, and then maintaining centerline “tracking” while data is written to or read from the disk. A closed loop servo system typically performs the seeking and tracking operations by controlling the rotary actuator in response to position information generated from the head.
A well known technique for generating the head position control information is to record servo information in servo sectors disbursed circumferentially about the disk, “embedded” with the data sectors. This is illustrated in FIG. 1 which shows a disk 2 comprising a number of concentric data tracks 4 and a number of embedded servo sectors 60-6N. Each servo sector 6i comprises a preamble 8, a sync mark 10, servo data 12, and servo bursts 14. The preamble 8 comprises a periodic pattern which allows proper gain adjustment and timing synchronization of the read signal, and the sync mark 10 comprises a special pattern for symbol synchronizing to the servo data 12. The servo data 12 comprises identification information, such as sector identification data and a track address. The servo control system reads the track address during seeks to derive a coarse position for the head with respect to the target track. The track addresses are recorded using a phase coherent Gray code so that the track addresses can be accurately detected when the head is flying between tracks. The servo bursts 14 in the servo sectors 6 comprise groups of consecutive transitions (e.g., A, B, C and D bursts) which are recorded at precise intervals and offsets with respect to the track centerline. Fine head position control information is derived from the servo bursts 14 for use in centerline tracking while writing data to and reading data from the target track.
The embedded servo sectors 6 are written to the disk 2 as part of the manufacturing process. Conventionally, an external servo writer has been employed which writes the embedded servo sectors 6 to the disks by processing each head disk assembly (HDA) in an assembly line fashion. The external servo writers employ very precise head positioning mechanics, such as a laser interferometer, for positioning the head at precise radial locations with respect to previously servo-written tracks so as to achieve very high track densities.
There are certain drawbacks associated with using external servo writers to write the embedded servo sectors 6 during manufacturing. Namely, the HDA is typically exposed to the environment through apertures which allow access to the disk drive's actuator arm and the insertion of a clock head which requires the servo writing procedure to take place in a clean room. Further, the manufacturing throughput is limited by the number of servo writers available, and the cost of each servo writer and clean room becomes very expensive to duplicate.
Attempts to overcome these drawbacks include a “self-servo writing” technique wherein components internal to the disk drive are employed to perform the servo writing function. Self-servo writing does not require a clean room since the embedded servo sectors are written by the disk drive after the HDA has been sealed. Further, self-servo writing can be carried out autonomously within each disk drive, thereby obviating the expensive external servo writer stations.
U.S. Pat. No. 5,949,603 discloses a technique for self-servo writing wherein the servo sectors are written relative to clock data disbursed around the disk and propagated from track to track. The clock data is first written to a “seed” track (e.g., at the inner diameter of the disk) from which the clock data as well as the servo sectors are propagated to the remaining tracks. The head is positioned over the seed track and, while reading the clock data in the seed track, the head is moved away from the seed track until the amplitude of the read signal decreases to some predetermined level. Then the clock data and servo sectors are written to the first track adjacent to the seed track. This process is repeated for the next and subsequent tracks until the embedded servo sectors have been written over the entire surface of the disk.
The radial offset between the read and write elements in magnetoresistive (MR) heads presents a problem when using the prior art technique of propagating servo sectors from track to track. Namely, when the head is positioned near the edge of the disk (inner or outer diameter depending on the geometry of the head) the resulting skew angle causes the write element to lag the read element in the direction of propagation. This lag renders it difficult for the read element to read the previously written servo track while writing the next servo track using the write element.
FIGS. 2A-2C illustrate another problem that manifests when servo writing the disk 2 using perpendicular magnetic recording. When writing the servo sectors 60-6N from the inner diameter of the disk (FIG. 2A) toward the outer diameter of the disk (FIG. 2C), the skew angle of the head's write pole 16 as it approaches the outer diameter causes the inner corner of the write pole 16 to “swing out” and overwrite a band 18 of the previously written servo data. Similarly, when writing the servo sectors 60-6N from the outer diameter of the disk (FIG. 2C) toward the inner diameter of the disk (FIG. 2A), the skew angle of the head's write pole 16 as it approaches the inner diameter causes the inner corner of the write pole 16 to “swing out” and overwrite a band 20 of the previously written servo data. The overwritten band (18 or 20) creates a “seam” between adjacent servo sectors, as well as a seam within each servo sector (including the servo bursts 14) if multiple revolutions are used to “stitch” together each servo sector 6i. The technique of “stitching” together a servo sector is typically necessary since the width of the write pole 16 is less than the width of a servo track requiring a portion (e.g., half) of a servo sector to be written during each revolution of the disk. The seams created by the overwrite problem illustrated in FIGS. 2A-2C induce errors in the position error signal generated when reading the servo bursts 14 as well as errors in detecting the servo data field 12, such as the Gray coded track addresses.
The prior art has suggested a number of techniques for addressing the overwrite problem when servo writing a disk drive using perpendicular magnetic recording. For example, U.S. Pat. No. 6,504,675 discloses a disk drive wherein the write pole has a trapezoidal shape in order to reduce the overwrite problem caused by the skew effect. However, the geometry of the trapezoidal shape varies between each disk drive due to tolerances in manufacturing the head, resulting in undesirable seams in the servo wedges for some percentage of the disk drives. In addition, manufacturing the write pole with a trapezoidal shape increases the manufacturing cost of the head, as well as reduces the surface area of the write pole leading to an undesirable decrease in the strength of the magnetic write flux.
U.S. Patent Application No. 2004/0061967 suggests an alternative solution to the overwrite problem by writing the servo sectors 60-6N from the outer diameter of the disk to the middle diameter, and then from the inner diameter to the middle diameter. A problem with this technique, however, is the seam created near the middle diameter of the disk where the two segments of a servo wedge “meet”. This seam becomes unusable (wasted) surface area, and the seek operation in the servo system must also account for the seam. This problem is exacerbated due to the disk expanding during the servo writing operation requiring a predetermined margin (wider seam) to account for the worst case deviation in the expansion.
There is, therefore, a need to overcome the problems associated with the skew angle of the head when servo writing a disk drive.